1. Field of the Invention
The invention relates to a method of managing the operation of a plant for the production of aluminium by igneous electrolysis. It also relates to the plant for implementing this method.
It relates more particularly to the management of the tools needed to operate such a plant, and especially to the lifting tools, handling tools, etc. conventionally employed in plants of the type in question.
2. Description of the Related Art
It is known that the production of aluminium by igneous electrolysis employs an alumina electrolysis reaction in a bath of molten cyrolite according to the reaction: EQU Al.sub.2 O.sub.3 +2e+Na.sub.3 [AlF.sub.6 ].fwdarw.2Al +3O.sub.2 +3C.fwdarw.3CO.sub.2 +F alumina cryolite
This reaction uses a molten bath comprising a mixture of cryolite and alumina, the temperature of which is generally greater than 800.degree. C. On account of the energy used and in order to limit the losses inherent in the start-up phases as far as possible, aluminium production plants employing this technology generally operate continuously with a series of aluminium tanks, the number and size of which depend, on the one hand, on the available amperage of the direct current supplying the tanks and, on the other hand, on the desired production quantity.
Moreover, plants are frequently organized in such a way that they have a number of tanks in series, mounted parallel to each other and installed within the same building or in separate buildings placed symmetrically with respect to a central passageway which is intended, in particular, to allow handling of the ladles containing a bath of molten cryolite for the electrolysis tanks, the beams for lifting the anode frames and other movements of the tank accessories, and which is more particularly intended for the handling of crucibles for tapping off the molten aluminium obtained by electrolysis.
Such series of electrolysis tanks may extend over relatively large distances, which may typically be as much as one kilometre, and, without this constituting a standard, a number of plants include two parallel series comprising 288 tanks, each of the series being served by eight identical complex machines which carry all the tools needed for the operation of said plant and which secure: the change of the anodes, which process includes:
the pricking of the surface crust forming on the PA1 upper surface of each of the tanks, that is to say PA1 the breaking of this crust, PA1 the collecting of all or some of the pieces coming PA1 from the breaking of said crust, PA1 the actual pulling-out of the spent anodes, PA1 the fitting of fresh anodes; PA1 the handling equipment; PA1 the mode of managing the plant; PA1 the design and engineering of the plant.
the lifting of the anode frames, this being necessary due to the wear of the said anodes over time; PA2 the operations of removing excess material from the bath of molten cryolite and of tapping off the molten aluminium obtained; PA2 and finally, the ancillary maintenance operations and corrective actions during operation.
Within plants known at the present time, the operating principle of which is illustrated diagrammatically with regard to FIG. 1, these machines (5) operate in a "slidewise" to-and-fro movement. FIG. 1 therefore shows four successive zones (11) representing four movements of each of the two machines (5) illustrated in this figure.
As indicated above, each of said machines (5) is capable of carrying out all the tasks corresponding to the functions needed for the correct operation of the plant. More specifically, a machine (5) carries out, during a first movement over a zone (11) of tanks to which it is assigned, one and the same defined task, so that it repeats this task at each tank. Said machine is then moved to the starting tank of the zone (11) in question so as to carry out a second task, different from the first, and the cycle is thus repeated for the various tasks necessary for the operation of the plant.
Thus, for the plant described above having 288 tanks, eight machines thus carry out the forward-and-return movements, and therefore the slidewise operation, in order for these various tasks to be accomplished.
Although, from the standpoint of the actual technical result, the mode of operation of this type of plant is satisfactory, it is accompanied, however, by drawbacks of a functional or structural nature which are recalled below.
Firstly, the tools of each machine have an insufficient utilization factor, consequently entailing an increased requirement in terms of machines. Moreover, because of their multitask function, these machines necessarily mean that the elements of which they are composed are overdesigned so as to be able to accomplish the tasks requiring high levels of technical performance and characteristics.
Next, this mode of operation leads to a complexity in the technical organization of the machines which is unfavourable to automation of the functions. This complexity of organization also results in the use of heavy and bulky machines, increasing the cost of manufacturing the machines, but also the more general civil engineering costs associated with the size of the buildings and of the factories in which such plants are mounted.
Finally, such an organization means that there have to be more operators, since it is necessary for someone to be in the cabin to control the machine and for someone to be on the ground to carry out the attendant manual operations, which furthermore involve safety problems.
In other words, the use of multifunctional or multitask machines limits the number of tanks capable of being treated by the same machine, given the operating process employed, in fact additionally increasing the operating costs of the plant in question.
This increase in the costs is therefore firstly inherent in the low utilization factor of the too numerous elementary functions for one and the same machine. It therefore concerns three main elements impacting the engineering of this type of plant, namely:
The so-called "slidewise" operation associated with such universal machines therefore involves, apart from the overall costs of very substantial and very heavy equipment, also very high running costs.